1. Field of the Invention
The present invention relates to a hydraulic control system in a continuously variable transmission as defined in the preamble of the following claim 1, in particular for use in a motor vehicle.
2. Description of the Related Art
Such a control system and transmission are known, for example from the European patent publication EP 1 939 503 A. The known continuously variable transmission includes a variator unit with a primary or drive variable pulley and with a secondary or driven variable pulley, as well as an endless flexible transmission element or drive belt, which may be one of several known types, wrapped around and in frictional contact with the said pulleys. The transmission further includes at least one clutch for engaging the transmission, i.e. for enable torque to be transmitted from an input shaft of the transmission to an output shaft thereof. Typically, the said clutch is part of an epicyclic or planetary gearing with one or more clutches and/or brakes for respectively engaging forward (drive) and reverse (drive), which gearing is also referred to as a DNR-set (Drive-Neutral-Reverse-set). Often, the transmission also includes a torque converter for amplifying a drive torque, in particular during the initial acceleration of the motor vehicle from standstill. The known torque converter is typically provided with a lock-up clutch that is engaged, i.e. closed sometime after the said initial acceleration to improve the transmission efficiency.
The transmission provides a speed ratio between the primary and secondary pulleys that may be controlled to an arbitrary value within a range of speed ratios covered by the transmission through an appropriate actuation of the said pulleys by means of the control system of the transmission. More in particular, each pulley comprises two sheaves where between the drive belt is held and whereof one sheave is arranged axially moveable along a respective pulley shaft, energized by the control system. To this end, the known control system includes two pressure cylinders, each associated with a respective one of the said moveable pulley sheaves. Further, the control system includes a primary valve for realizing, in a controlled manner, a pressure level in the pressure cylinder associated with the primary pulley and a secondary valve for realizing, in a controlled manner, a pressure level in the pressure cylinder associated with the secondary pulley. These cylinder pressures determine the clamping forces respectively exerted on the drive belt between the sheaves of each pulley and as a consequence the said speed ratio, as well as the torque that can be transmitted by the transmission.
The control system also includes a pump for supplying the control system with a flow of hydraulic fluid to a main line of the control system. The hydraulic pressure in the main line, i.e. the line pressure, is regulated by means of a controllable line pressure valve of the control system. From this main line the said pressure cylinders are fed with hydraulic fluid under the control of the said primary and secondary valves.
Any surplus of the pump flow is discharged by the line pressure valve into an auxiliary line of the control system, wherefrom said at least one clutch of the transmission can be supplied with hydraulic fluid for the selective engagement, i.e. closing, thereof. The hydraulic pressure in the auxiliary line, i.e. the auxiliary pressure, is set by means of an auxiliary pressure valve of the control system to a pre-determined, i.e. fixed level that is sufficient for engaging the clutch to the extent that it is capable of transmitting the maximum torque to be transmitted. Typically, an auxiliary pressure level of around 12 bar is required for this purpose, as compared 9 to a maximum line pressure level somewhere in the range from 25 to 80.
Because of this known arrangement of the control system, wherein the auxiliary line is located downstream of the main line, the line pressure is controlled to be higher than or at least equal to the auxiliary pressure at all times. According to the present invention this known arrangement comes with the disadvantage that the cylinder pressures required for transmitting the torque to be transmitted can be lower than the auxiliary pressure, but that the line pressure can not. More in particular, it is generally true that the cylinder pressures are dependent on both the torque to be transmitted and on the transmission ratio, whereas the clutch engagement pressure is primarily dependent on the torque to be transmitted. In other words, in certain operating conditions of the transmission, in particular in an accelerating transmission speed ratio and/or at a low torque to be transmitted, the line pressure level that would, theoretically, be required for generating the cylinder pressures is lower than the line pressure level that is actually required for generating the said fixed auxiliary pressure level.